Why Does TDS Rise When Your RO System Is Idle?

A reminder: TDS (Total Dissolved Solids) measures the amount of dry residue in a liquid. It is particularly useful for checking the effectiveness of a filtration system, especially a reverse osmosis system designed to remove as many impurities from the water as possible.

If you have a reverse osmosis system with an LCD screen, you have probably noticed that when the pump is off, the TDS displayed on the screen gradually rises, sometimes exceeding 100PPM.

This phenomenon is completely normal: it is natural osmosis.

Here is how it works: osmosis balances the concentration and pressure of two solutions separated by a semipermeable membrane, which means that it allows only the liquid through, but not the solids. If you pour an almost pure liquid into one side of a U-shaped tube fitted with this type of membrane and a highly concentrated liquid into the other, the level on the side with the lower concentration will naturally fall as the level on the side with the higher concentration rises.

Reverse osmosis explained

A reverse osmosis system uses the opposite phenomenon by applying pressure on one side of the membrane to force the mineral concentrations as far apart as possible. This makes it possible to obtain almost pure RO water and drain water with a very high concentration of impurities.

However, when the pump is not running and the pressure balances on both sides of the RO membrane, the mineral concentrations also tend to balance, which is why TDS rises. There is no need to worry: this high TDS affects only the first few cl of water at your RO system's outlet. You will see TDS drop quickly as soon as the pump starts running again.

For more information about reverse osmosis systems, contact us at 09.72.55.18.50 or through the contact form.

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REVERSE OSMOSIS

Why Does TDS Rise When Your RO System Is Idle?

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